Morphological and structural analysis of dialcohol cellulose fibres: a multi-length scale approach

Licentiate thesis, 2024

There is an urgent need for a transition from the consumption of fossil-based resources to sustainable and renewable counterparts. Bio-based alternatives like cellulose are promising materials to replace synthetic polymers. However, cellulosic materials require some chemical modification to facilitate processing into complex structures. One way to overcome this challenge is to partly modify cellulose in cellulose fibres to dialcohol cellulose, which can be softened by both temperature and moisture. This thesis explores the swelling behaviour of partially modified dialcohol cellulose fibres (bleached softwood kraft pulp) of different degrees of modification (DOM); from the nanoscale to the microscale using characterisation tools such as x-ray scattering and optical microscopy. Wide-angle x-ray scattering reveals the structures inside the fibre wall at the Angstrom level, while small-angle x-ray scattering (SAXS) provides information about the morphologies at the nanometre level. The SAXS results give an estimation of the distance between nanofibrils and the swelling of nanofibril bundles. The SAXS results show that with an increase in DOM, the SAXS peak shifts from ~5 nm (for unmodified, bench-dried cellulose fibres) to ~11 nm (for bench-dried, dialcohol cellulose fibres with 51% DOM). In addition, at the fibre level, microscopic studies indicate that when fibres are partially modified, a mild ballooning in the swollen fibres is observed (for never-dried fibres of ~25% DOM) with an average fibre width of 35 ± 9 µm to 58 ± 24 µm (for never-dried fibres of ~50% DOM). Furthermore, the water retention value ranged between 1.7˗6.3 g-water/g-fibre, depending on the degree of modification. Interestingly, there was a linear correlation between the swelling propensity from the water retention values and the nano-swelling from the SAXS peaks, indicating that the SAXS peaks correlate to the microscopic swelling of the fibre wall.